1. Field of the Invention
The present invention relates to a carbon nanotube device which contains at least a carbon nanotube, and to a method of manufacturing such a carbon nanotube device. More particularly, the present invention relates to a method of manufacturing a carbon nanotube device including connecting internal electrodes in the device to a carbon nanotube, and to a carbon nanotube device manufactured using such a method.
2. Description of the Related Art
Fibrous carbons are generally called carbon fibers, and conventionally, several kinds of methods of manufacturing carbon fibers having a thickness of several μm or more in diameter used for structural materials have been studied. At present, of those, the method of manufacturing carbon fibers from polyacrylonitrile (PAN)—or pitch-based materials is the most widely used.
Apart from this, the carbon nanotubes discovered recently are made of a tubular material with a thickness of 1 μm or less in diameter. Ideally, a carbon face of a hexagon mesh forms a tube in parallel to an axis of the tube and plurality of the tube may be multi-layered. It is theoretically estimated that the carbon nanotubes have either a metallic or semiconductor property depending on how carbon hexagon meshes are linked and the thickness of the tubes, allowing expectation that it will be a promising functional material.
Usually, to synthesize the carbon nanotubes, an arc discharge method is used and in addition, the methods including a laser evaporation method, a pyrolytic method, and a method using chemical vapor deposition have recently been studied. The carbon nanotubes recently developed are generally described below.
(Carbon Nanotube)
The material with a diameter of 1 μm or smaller which is finer than carbon fibers, is generally called carbon nanotubes and distinguished from the carbon fibers, although there is no particularly definite boundary therebetween the both types of carbon fibers. By a narrow definition, the material, of which carbon faces with hexagon meshes are almost parallel to the axis of the tube, is called a carbon nanotube and even a variant of the carbon nanotube, around which amorphous carbon exists, is included in the carbon nanotube. (Note that with respect to the present invention, this narrow definition is applied to the carbon nanotube.)
Usually, the narrowly-defined carbon nanotubes are further classified into two types: carbon nanotubes having a structure with a single hexagon mesh tube are called single wall nanotubes (hereafter, simply referred to as “SWNT” in some cases; and the carbon nanotubes made of multilayer hexagon mesh tubes are called multi-wall nanotubes (hereafter, simply referred to as “MWNT” in some cases). Type of carbon nanotubes may be determined depending on how to synthesize and the established conditions to some degree but production of purely one type of the carbon nanotubes has not yet been achieved.
The carbon fibers have larger diameters and incomplete cylindrical mesh structures parallel to the axes of the tubes. The nanotubes produced by a vapor-phase pyrolysis method using a catalyst have a tubular mesh structure parallel to the axis of the tube in the vicinity of a center of the tube and in many cases, a large amount of carbon having a disordered structure surrounds it.
Now, the arc discharge method as a typical method of manufacturing carbon nanotubes, will be briefly described below.
The arc discharge method, which was first discovered by Iijima, is described in detail in “Nature” (Vol. 354, 1991, p 56 to 58). The arc discharge method is a simple method, by which direct current arc discharge is performed using carbon electrode rods in an atmosphere containing argon under about 13300 Pa (100 Torr). The carbon nanotubes grow together with carbon particles of 5 to 20 nm in size in a partial area on a surface of a negative electrode. The resultant carbon nanotubes have a layer structure, in which tubular carbon meshes with a diameter of 4 to 30 nm and a length of about 1 to 50 μm are overlapped, the mesh structure of carbon being helically formed in parallel with its axis.
Helical pitches vary depending on tubes or layers in the tube and for multilayer tubes, the distance between the layers is 0.34 nm, which is almost identical to the distance between graphite layers.
Note that, carbon nanotubes have high electrical conductivity and when an attempt is made to apply the carbon nanotubes to electronic devices (hereafter, in some cases, simply referred to as “devices”), they must be connected to electrodes.
When carbon nanotubes are arranged between a pair of electrodes and the electric resistance across the resultant structure is measured, in many cases, resistance is measured higher than that expected from a high electrical conductivity of a carbon nanotube itself. This is considered to be due to a contact resistance etc. generated between the carbon nanotube and the metallic electrodes. For industrial application of carbon nanotubes, it is extremely important to realize ohmic contacts between the carbon nanotubes and the electrodes, with the contact resistance being reduced.
Gold (gold pad) is widely used as an electrode material for carbon nanotubes. This gives an ohmic contact or no ohmic contact according to the cases; it gives poor reproducibility.
Examples of the method of realizing an ohmic contact between the carbon nanotubes and the metallic electrodes include the following.
(1) A method in which hydrocarbon is graphitized and the resultant is placed between the electrodes and the carbon nanotubes.
(2) A method in which carbon nanotubes are arranged on a gold pad and the joined part is irradiated with electron beams (cf., Applied Physics Letters, 1998, Vol. 73, 274).
(3) A method in which Au/Ti is used as an electrode material (cf., Applied Physics Letters, 1999, Vol. 75, 627).
(4) A method in which a transition metal which readily gives an ohmic contact, such as Sc, Ti, or V, is used as an electrode material (cf., Applied Physics Letters, 2000, Vol. 76, 3890).
(5) A method in which the joined part between the electrode part and the carbon nanotubes are positively chemically combined by heating:
a) It has been confirmed that when Si or a transition metal (Ti or the like) and a carbon nanotube is joined and heated, the Si or transition metal reacts with the carbon of the carbon nanotube to generate as a compound therebetween, a carbide (silicon carbide, or metal carbide). On this occasion, the carbon nanotube and the carbide are joined smoothly in the atomic level to give a better electrical connection therebetween (cf., Science, 1999, Vol. 285, 1719).
b) It has been confirmed that when niobium (Nb) and a carbon nanotube are joined and heated to 950° C., the joined part is converted into niobium carbide as a compound therebetween (cf., Applied Physics letters, 2000, Vol. 77, 966).
c) It has been confirmed that through heat treatment at 800° C. performed on a device including a Ti/Au electrode with which a carbon nanotube is joined so as to convert the joined part into titanium carbide, in some cases, an ohmic contact between the both is achieved (cf., Journal of Physics D, 2000, vol. 33, 1953). In addition, in Physical Review Letters, 2001 vol. 87 256805, it was reported that analysis by X-ray diffraction revealed formation of stable TiC at 800° C. or higher.
As described above, various methods have been considered to realize ohmic contact between the carbon nanotube and the electrodes. However, none of them were easy to realize a complete ohmic contact, with the reproducibility of ohmic contact being insufficient. In the method of positively chemically combining the joined part between the electrode part and the carbon nanotubes by heating, heating cannot be performed at so high a temperature because the carbon nanotubes themselves should be destroyed due to the heat; also realization of complete ohmic contact is difficult to achieve and the reproducibility of ohmic contact has been insufficient.